Offshore storage tank system



Jan. 21, 1969 c. A. M DONALD 3,422,523

' OFFSHORE STORAGE TANK SYSTEM Filed Aug. 26, 1966 Sheet of e INVENTOR @fia Q BY A TTORNEXS Jan. 21, 1969 c. A. M DONALD 3,422,628

OFFSHORE STORAGE-TANK SYSTEM Filed Aug. 26, 1966 Sheet g of 6 INVENTLOR ,w yg

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A TTORNE'YS 1969 c. A. MCDONALD 3,422,628

OFFSHORE STORAGE TANK SYSTEM Filed Aug. 26, 1966 Sheet of e ATTORNEYS Jan. 21, 1969 c. A. M DONALD 3,422,623

OFFSHORE STORAGE TANK SYSTEM Filed Aug. 26, 1966 Sheet '1 of 6 INVENTOR.

Jan. 21, 1969 c. A. M DONALD 3,422,528

OFFSHORE STORAGE TANK SYSTEM Filed Aug. 26, 1966 Sheet 6 of 6 /62 /54 ilk 5 I20.

INVENTOR.

' ATTORNEYS United States Patent 3,422,628 OFFSHORE STORAGE TANK SYSTEM Charles A. McDonald, Palos Heights, lll., assignor to Chicago Bridge & Iron Company, Oak Brook, 11]., a corporation of Illinois Filed Aug. 26, 1966, Ser. No. 575,272 US. CI. 6146 Int. Cl. E02d 21/00, 17/00; F17d 1/08 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to an offshore storage tank systern for storing large quantities of fluids in the vicinity of underwater wells for subsequent transportation to land. More particularly, the invention relates to a system incorporating a plurality of storage tank units in a modular construction.

Large quantities of hydrocarbons are obtained from producing wells beneath inland or ocean waters. In many cases, it is necessary or desirable that the hydrocarbons be stored adjacent to the wells for intermittent removal and transport by barges or tankers, without interrupting oil production. In such cases, storage facilities must be pro vided, and it is highly desirable that the facilities be capable of storing large volumes of hydrocarbons, so that the hydrocarbons may be removed intermittently and transported by large barges or tankers at minimum cost.

In addition to the storage facilities, it is necessary that transfer equipment be provided, and it may be necessary or desirable to provide processing equipment. Provision must be made to accommodate personnel on either a full time or a part time basis.

In many offshore locations, especially in water depths from 50 to 300 feet, severe storms may occur, with accompanying high winds and large waves, which cancause great damage to floating and submerged structures. It is essential that the storage structures employed in such locations be capable of withstanding the natural forces of wind, waves and gravity that are encountered.

The storage capacity requirements vary from one location to another, necessitating the construction of storage facilities of various sizes, or suffering disadvantageous over capacity or under capacity. In the former case, the excess capacity represents a waste of structural materials and excessive cost, and the problems of transporting,

I erecting, securing, and removing the storage facilities are greater than they need be. In the latter case, the hydrocarbons must be transferred to the transportation vessels at slow rates and transported in relatively small quantities, rendering the operations uneconornical.

The present invention provides an offshore storage tank system of modular construction wherein the modules are self-supporting storage tank units each including a pedestal and a storage tank mounted on the pedestal. Lateral support is provided by internal tension and compression members that interconnect the pedestals in a unitary storage structure having the strength and rigidity necessary for stability when exposed to severe natural forces of wind,

3,422,628 Patented Jan. 21, 1969 waves and gravity. Also provided are filling and emptying systems for storage tanks.

The modular construction provides for flexibility in construction to accommodate different required storage capacities and also to enable the construction of unitary storage structures having the most desirable configurations for their respective needs. This can be accomplished by varying the size, the spacing, the number or the type of storage tank units or by a combination of these variations. The storage structures are constructed of at least three storage tank units, constituting the modules, which can be disposed around a normally vertical axis or they can be arranged in any non-aligned symmetrical or nonsymrnetrical grouping, to provide structural stability for withstanding the natural forces. Inasmuch as each storage tank unit is self-supporting, it is only necessary to provide lateral support for integrating the storage tank units into a storage structure that withstands horizontal forces as wellas vertical forces. The individual storage tank units may be constructed in conventional ways for such units employed at onshore locations.

The storage tanks of the new storage structure are supported above the water level to provide large volume elevated storage capacity. The pedestals may provide additional storage capacity, both elevated and submerged below the water level. Further storage capacity may be provided by one or more hollow bodies mounted on the structure below the water level. In a specific embodiment of the invention, a hollow body is mounted on the storage structure at points normally below the water level, for imparting buoyancy to the structure. The storage tanks and the hollow body then may impart suflicient buoyancy and stability to permit towing the structure in the water from one location to another. When the unit is installed, the hollow body may provide the aforesaid additional submerged storage capacity.

A platform advantageously is mounted on the unitary storage structure. The platform may serve to accommodate transfer equipment, processing equipment, and personnel, and provide space for a heliport. Also, a platform may be mounted on two or more unitary storage structures to extend therebetween.

The storage structure, and a hollow body and a platform thereon, may be anchored to a foundation resting on the bed of the body of water, to secure the system rigidly in a fixed location, where it 'will remain as a stable unit so long as desired. Provision may be made for removing the structure from the foundation when it is no longer needed at that location, so that it can be moved to another location and reused at such location.

The invention provides large volume storage capacity most economically, by utilizing storage tank units or modules of proven design which can be constructed at minimum cost. The storage tank units are efiiciently, economically, and reliably interconnected in a strong and rigid unitary storage structure. The structure is well adapted for transportation to a producing location, and from one location to another. The structure may be installed readily at each location.

These and other features, objects, and functions of the invention will be apparent on reference to the specification and to the attached drawings illustrating preferred embodiments of the invention, wherein like parts are identified by like reference symbols in each of the views, and wherein:

FIGURES 14 are side elevational views of four different illustrative embodiments of the storage tank unit of the invention;

FIGS. 5-7 are schematic elevational views of three different embodiments of the storage tank system of the invention;

FIGS. 8-10 are schematic top plan views of the embodiments of FIGS. -7, respectively;

FIGS. 11-13 are schematic horizontal cross sectional views of three additional embodiments of the storage tank system, illustrating particularly the arrangements of plat forms thereof;

FIGS. 11 and 12 illustrate the provision of a plurality of unitary storage structures bridged by platforms;

FIG. 14 is a side elevational view of an embodiment of the storage tank system that incorporates a storage tank unit similar to that of FIG. 2, in an arrangement similar to that of FIGS. 5 and 8. FIG. 14 illustrates the manner in which the structure may be mounted on the bed or floor of a body of water, and also, the provision of a submerged hollow body on the structure;

FIG. 15 is a top plan view of the structure of FIG. 14;

FIG. 16 is a schematic elevational view showing a piping system for filling and emptying a storage tank system;

FIG. 17 is a schematic elevational view showing an alternative piping system for filling and emptying a storage tank system; and

FIG. 18 is a schematic elevational view showing still another alternative piping system for filling and emptying a storage tank system.

FIGS. 5l5 illustrate :various embodiments of the offshore storage tank system of the invention. The system incorporates one or more unitary storage structures 20, 20a, 22, 24, or 26. Each of the storage structures is a modular construction including a plurality of at least three self-supporting storage tank units or modules 28, 30, a, 32, or 34. The unit 30 of FIG. 2 is employed in the illustrative structures of FIGS. 5-13, and a similar unit 30a is employed in the structure of FIGS. 513, and FIGS. 14 and 15. As indicated in FIGS. 8-10, 13 and 15, the storage tank units are disposed around a normally vertical axis 36, 38, or 42 or are arranged in a nonaligned grouping.

The storage tank units 28, 30, 30a, 32, and 34 each include a tubular or hollow cylindrical pedestal or column 44, 46, 46a, 48, or 50. The pedestal normally supports the unit on the bed or floor 52 (FIG. 14) of a body of water 54 having a water level 56, with the pedestal vertical or inclined. Each storage tank unit also includes a hollow storage tank 58, 60, 62, or 64 integrally mounted on the pedestal. The storage tank normally is supported by the pedestal above the water level 56 and above the reach of the highest waves.

The storage tank 58 of FIG. 1 is generally cylindrical with a hemispherical or rounded upper end 66. The tank includes integrally joined tapering transition elements 68 and 70. The lower element 70 is integrally joined to the upper end of the pedestal 44, in open communication therewith or isolated therefrom by a suitable partition as desired.

The storage tank 60 of FIGS. 2 and 14 is generally spherical. It includes an integrally joined tapering transition element 72. The element is integrally joined to the upper end of the pedestal 46 (FIG. 2) or the pedestal 46a (FIG. 14) in open communication therewith 0r isolated therefrom by a suitable partition as desired.

The storage tank 62 of FIG. 3 is generally ellipsoidal. It includes integrally joined tapering transition elements 74 and 76. The lower element 76 is integrally joined to the upper end of the pedestal 48, in open communication therewith or isolated therefrom by a suitable partition as desired.

The storage tank 64 of FIG. 4 is generally cylindrical, and it has an open upper end 78. The tank includes integrally joined tapering transition elements 80, 82, and 84. The lower element 84 is integrally joined to the upper end of the pedestal 50 in open communication therewith or isolated therefrom by a suitable partition as desired.

A floating roof 86 is mounted within the tank.

The storage tank units 28, 30, 32, and 34 represent conventional units employed for fluid storage on land. They are constructed in conventional ways, with the parts secured together by suitable means, such as welding. It is a feature of the invention that these and other conventional units employing the illustrative and other suitable tank configurations may be utilized in the offshore storage tank system. Thus, the design parameters are known, and construction is most eflicient and economical.

Three or more of the units such as illustrated in FIGS. 1-4 and 14 are grouped in one of the arrangements illustrated in FIGS. 510 and 1315, or other appropriate numbers and groupings of the units may be employed. It will be evident that for optimum structural balance, it is preferable to employ one type of storage tank unit in each of the storage structures 20, 20a, 22, 24, and 26, or other structure. However, if desired, more than one type of unit may be employed in a storage structure with suitable provision for the variation in design and/or capacity.

The illustrative storage structure have triangular, rectangular, pentagonal and hexagonal configurations, as viewed in plan. The storage tank units 30 and 30a, or other units such as 28, 32, and 34, preferably are equidistantly spaced from the vertical axes 36, 38, 40 and 42, and from each other, whereby their centers or longitudinal axes are spaced equiangularly around the circumference of circles having their centers coincident with the axes of the storage structures. Also, the storage tanks 60, or other tanks, preferably are supported at the same elevations in each of the storage structures. Such arrangements of the storage tank units in the structures provide optimum structural balance. However, the storage tank units may be arranged in the structures in other ways, if desired.

As illustrated in FIGS. 510, 13, 14 and 15, the storage tank units 30 and 30a, or other units 28, 32, or 34, are interconnected in the unitary storage structures 20, 20a, 22, 24, and 26. In particular, pedestals 46 and 46a, or other similar pedestals 44, 48, or 50, are interconnected with a plurality of horizontal struts or cross braces 88 and a plurality of inclined struts or cross braces 90. The struts constitute internal tension and compression members interconnecting the storage tank units in a strong and rigid structure. The struts may be tubular structural members, as illustrated, angle bars, or other structural mernbers, which are secured to the pedestals by welding or other suitable means.

In the embodiment of FIGS. 14 and 15, an annular externally sealed hollow body or buoyancy chamber 92 in the shape of a triangle is secured to the pedestals 46a, as by welding, and the hollow body provides strut means interconnecting the lower ends of the pedestals. The hollow body serves to impart buoyancy to the structure, and it also provides additional storage capacity. The body may be open throughout its interior or may be separated into compartments.

The storage tanks 60, the pedestals 46a, and the hollow body 92 of FIGS. 14 and 15 are externally sealed or liquid tight and the interiors of all are used as storage volumes. In one embodiment, all these volumes in a unitary storage structure are in open communication to provide a single storage volume. Alternatively, individual storage tanks 60, pedestals 46a, or portions of the hollow body 92 may be isolated by means of suitable partitions to provide flexibility in storage or operation as when processing equipment is installed on the storage tank system. In any embodiment, the strut members 88 and 510, if tubular or hollow, may have their internal volumes in open communication with the internal volume of the members to which they attach in order to provide increased storage volume. Alternatively, some or all of the hollow struts 88 and 90 or extensions thereof may he used to interconnect such isolated storage volumes as storage tanks 60, pedestals 46a, the hollow body 92, or portions thereof, in various combinations as may be advantageous.

In storing hydrocarbons or other fluids in the storage structure 20a (FIG. 14) or other structure, the liquid displacement principle of operation is employed. There must be at all times sufiicient liquid within the structure, properly distributed, to overcome the upward buoyant forces due to immersion in the water, and the upward forces in the structure caused by the action of wind and water forces.

The maximum safety against displacement of the structure due to wind and water forces is obtained when all storage volumes are maintained full of liquid, either hydrocarbons, water, or both. In one embodiment, wherein all the storage volumes in a unitary storage structure are in open communication to provide a single storage volume, the manner of operation and exemplary equipment to maintain the storage volume full of liquid are described in the application of Robert S. Chamberlin, Charles A. McDonald and Donald C. Stafford, Ser. No. 448,947 filed Apr. 19, 1965.

FIGURE 16 illustrates an advantageous alternate meth- 0d of filling and emptying a unitary storage structure, similar to that shown in FIGURES 14 and 15, wherein all the storage volumes are in open communication to provide a single storage volume, in a manner which reduces the cost of pumping fluids, while maintaining adequate stability against buoyancy and the wind and wave forces.

Referring to FIG. 16, a cycle of fill and empty operations will the described starting with the storage volume filled with water to the elevation of the external water level 56. Hydrocarbons are transported by other means from a wellhead at the bottom of the body of water, through pipe 120 to the processing equipment 122 located upon the platform 102, and is then transferred to the storage volume. Valves 126, 130, 168 and 140, lines 128, 172 and 124 and pumps 132 and 138 are all advisably located on platform 102 as are the same elements of FIGS. 17 and 18.

With reference to FIG. 16, initially all valves are closed and all pumps off. The filling operation is started by opening valves 126 and 162, thus permitting hydrocarbon to flow by gravity or due to the pressure in the processing equipment 122 into the storage volume via pipe 128, which may be connected with the hollow pedestals 46a or alternatively with one or more of the horizontal hollow struts 88 either above or below the external water level 56, thus displacing air which escapes through the vent 174 at the top of each unitary storage structure 20a, and water which flows out pipes 158 and 1-60 and discharges through valve 162 located preferentially slightly above the external water level 56 to provide a visible discharge.

This naturally occurring, gravity induced, operation continues without any external pumping being required until the incoming hydrocarbon has displaced essentially all the water from inside the storage volume, and the hydrocarbon-water interface has been lowered to the elevation 164 of the bottom of the hollow body 92, at which point a dielectric strength detecting device 176, or other suitable device, detects the change from water to hydrocarbon at elevation 164 and by suitable means causes valve 162 to close, thus stopping the outflow of water from the storage volume.

As the inflow of hydrocarbon continues, the hydrocarbon-air interface elimps up the pedestals 46a and at a suitable time determined by the available pressure of the hydrocarbon at the platform elevation 102, valve 130 may be opened, pump 132 started, valve 126 closed, and the hydrocarbon supplied via pipes 134 and 128 into the storage volume, as before, at higher pressure and rate of flow, until the storage volume is filled with hydrocarbon to elevation 166, at which point pump 132 is stopped and valve 130 is closed by a float gauge or other suitable means located at elevation 166.

To withdraw hydrocarbon to fill a tanker or barge,

valve 168 is opened to allow the hydrocarbon to flow under gravity through pipe 128, valve 168 and through pipe 170 into a tanker or barge until such time as the flow rate becomes inadequate due to reduced gravity pressure. When this occurs, pump 138 is started, valve is opened and valves 130, 126 and 168 closed. Hydrocarbon flows from the storage volume, through pipes 128 and 136, pump 138, valve 140 and pipe 172, discharging with increased flow through pipe into a barge or tanker.

During the barge or tanker filling operation, whether by gravity or by forced pumping, hydrocarbons from the wellhead at the floor of the body of water or from the processing equipment 122 can be accepted or received, without interruption, by operating pump 132, opening valve 130, and allowing the hydrocarbon to flow either through pipe 134, valve 168 and pipe 170 as when filling the tanker by gravity, or through pipe 136 and to the forced pump circuit 138, 140, 172 and 170 to the tanker, or into the storage volume via pipes 134 and 128, depending upon the pressures and flows existing at the time.

During the emptying of hydrocarbons from the storage volume, air enters through the vent 174 located above the upper liquid level 166. The hydrocarbon-air interface falls gradually until it reaches the elevation of the discharge from pipe 170, at which time gravity outflow would cease; or, if pump 138 is being used to discharge hydrocarbon, the hydrocarbon-air interface Would fall gradually until it reached an elevation at which the pump 138 would stop operating due to the lack of an adequate pressure in the supply pipe line 136. To prevent this undesirable cessation of hydrocarbon flow before the storage structure is empty of hydrocarbon, pump 152, located either on platform 102 or on a separate platform near the elevation 56 of the external water, or submerged as may be desired, and capable of pumping water through the pipe 150, valve 154 and pipes 156 and 158 into the storage volume, is provided. As the hydrocarbon emptying proceeds and the hydrocarbon-air interface drops, at an appropriate time pump 152 is started, valve 154 is opened, and water pumped through pipes 150, 156 and 158 into the bottom of the storage volume, thus displacing the hydrocarbon and forcing the water-hydrocarbon interface up through the bottom hollow body 92 and up the pedestals 46a. Pump 152 must have sufiicient capacity and be started early enough so that the air-hydrocarbon interface is prevented from falling below that elevation at which either the gravity hydrocarbon discharge rate is seriously reduced, or that at which the forced discharge through pump 138 is seriously reduced. It is apparent that the capacity required in pump 152 can be substantially less than the desired discharge capacity through pipe 128, because pipe 128 can be supplied by hydrocarbon flowing downward under the force of gravity from the storage tanks 60, and simultaneously by hydrocarbon being displaced upward by incoming water from pump 152.

A dielectric detecting device 177, located in one of the pedestals 46a slightly below the elevation of the hydrocarbon inlet-outlet pipe 128, detects the approach of the hydrocarbon-water interface to the outlet line 128 and can, in a manner known to those skilled in the art, initiate the closing of valve 168, the stopping of pump 138, the closing of valve 140, the stopping of pump 152 and the closing of valve 154, as may be appropriate in the circumstances. Alternately, a float means, with suitable guides, can be used to indicate the elevation of the hydrocarbon-water interface in the pedestals 46a, and to actuate valves and pumps in a manner to prevent the outflow of water through the hydrocarbon outlet pipe 128 and the inflow of water through pipe 158. Control means are provided for valve 162 such that it is normally closed, but always closes and remains closed when hydrocarbon is detected at elevation 164 by the dielectric detecting device 177, and can be opened under manual control provided no hydrocarbon is detected at elevation 164. By

control of the hydrocarbon inflow via pipe 128 and water inpumping by pump 152, the liquid-air interface within the storage volume may be raised to, or maintained at, any desired elevation.

One of the objects of this invention is to reduce the energy required to fill and empty the storage volumes of hydrocarbon or water as the case may be. It is well known in the art that submerged storage structures can be filled and emptied of hydrocarbon and water by means of compartments and/ or pipes connecting to the storage volume at locations substantially above the normal water surface level and above the topof the storage volume. Such locations are selected to provide an internal hydraulic pressure which at all points within the storage volume exceeds that exerted by the surrounding water in which it is immersed; or to provide a height of hydrocarbon sufliciently greater than the balancing head of water that, having regard for their different specific gravities, the pressure exerted by the hydrocarbon will be great enough to displace the water from the storage volume, thus causing it to be filled with hydrocarbon. It is a feature of this invention that the hydrocarbon inlet-outlet connection 128 to the storage volume can be located either above, at, or below the normal water elevation 56, in a manner independent of the relative specific gravities of the fluids involved. It is a further feature of this invention that the water required during the filling or emptying of the storage volume with hydrocarbon need not at any time be elevated above the normal water surface level 56, although for operating convenience the pumps 152 and the valves 154 and 162 may be so located if desired. In other words, the water is not lifted above the water-hydrocarbon interface in the storage volume to achieve displacement of the hydrocarbon therefrom. If a severe storm is expected, and it is desired to have more liquid within the storage volume in order to increase the stability of the unitary storage structure, water may be introduced by means of pump 152, valve 154, and pipes 156 and 158 to raise the liquid level within the storage volume to the desired elevation.

The elevation or location of the connection of the hydrocarbon inlet-outlet pipe 128 to the storage volume in relation to the normal water surface elevation 56 has a significant effect upon the energy required for filling and for emptying the storage volume with hydrocarbon even though the connection can be above, at, or below the normal water elevation. When the inlet-outlet pipe 128 is connected to the storage volume at or near the top liquid elevation 166, a maximum of energy is required for filling and also for emptying operations since all of the hydrocarbon entering or leaving the storage volume must be lifted to this elevation. As the elevation of the connection of the inlet-outlet pipe 128 to the storage volume is lowered from elevation 166 toward the elevation 56 of the normal water surface, the amount of energy required for each filling and each emptying of the storage volume with hydrocarbon diminishes in a manner that may be precisely calculated.

If the storage tank system is provided with pumping facilities located on a platform above the normal water surface elevation 56, and it is desired to have inflowing and outflowing hydrocarbons pass through such equipment, there is no saving in energy, either during the filling or the emptying of the storage volume with hydrocarbon, in having the inlet-outlet pipe 128 connection to the storage volume located below the elevation of the platform. When such pumping facilities are not provided, a minimum of energy is required for each of filling and emptying the storage volume with hydrocarbon when the inlet outlet pipe 128 is connected to the storage volume at or near the normal water surface elevation 56.

It is apparent that the single inlet-outlet pipe 128 may be replaced by a separate inlet pipe 178 and an outlet pipe 179 located at essentially the same elevation, or at different elevations as shown in FIGURE 17 to obtain different operating characteristics that may be desirable in certain circumstances. The piping arrangement shown in FIGURE 17, by lowering the connection of the discharge pipe 179 closer to the mean water surface elevation 56, both reduces the length of discharge piping and increases the discharge rate by reducing the friction loss in the pipe.

It will be apparent that the arrangements of filling and emptying equipment, illustrated in FIGURES 1 6 and 17, by permitting many combinations of gravity and forced pumping flow rates, together with different durations of gravity and forced flows of both water and hydrocarbons, provides great flexibility in the selection of equipment and operating procedures which will permit the optimizing of the initial capital expenditure, operating cost, operating characteristics, or any desired combination, thereof.

In many situations the collection of sediments in the lower portions of the storage volume pose problems for their removal and disposal. FIGURE 18 illustrates one embodiment of this invention wherein the upper storage tank 60 of one or more of the storage tank unit modules 20a is utilized as both a settling basin and a storage volume by the provision of bulkhead 182 and overflow pipe 183 which penetrates the bulkhead and extends upward. The length of pipe 183, in conjunction with the dimensions of the storage tank 60, and the number of tanks involved, determines the volume of the settling basin and hence the average detention time of the incoming hydrocarbon. The elevation of the top of overflow pipe 183 also determines the minimum height to which incoming hydrocarbon must be lifted and hence the energy required to fill the storage volume with hydrocarbon. Incoming hydrocarbon is discharged via pipe 178, at an elevation below the top of pipe 183, and at a suflicient height above baflle 182 to avoid disturbing the settled sludge in the bottom of storage tanks 60 which provides a sufficient detention time for the sediments to settle to the bottom. The sediment free hydrocarbon overfiows the pipe 183 which communicates directly with the pedestal 46a, and thence by means of struts 88 and 90 and the hollow body 92 with all other portions of the storage volume. Thus, all sediment is retained in one or more of the upper storage volumes 60 and, with valve 168 closed, may be discharged through pipe 185, valve 184, pipe 136, pump 138, valve 140, and pipes 172 and 170 into a barge or tanker for removal. With valves 184 and 168 open, and valve closed, simple gravity drainage is also possible. Normally this operation coincides with the removal of hydrocarbon from the other upper storage tanks 60 via outlet pipe 179 in order to maintain a close balance between the pressures above and below baflle 182.

This invention thus provides the method of filling an offshore hollow storage structure, which is partially above and partially below a sea level, with a liquid hydrocarbon less dense than water, which comprises pumping a liquid hydrocarbon into the storage structure by means of a feed line communicating with the storage structure at from about sea level upwardly to substantially below the top of that portion of the storage structure above sea level to displace water from the submerged portion of the storage structure, and continuing pumping liquid hydrocarbon into the storage structure through such feed line until the storage structure above sea level is filled at least to substantially above the feed line and up to the top of the storage structure, with that portion of the storage structure above sea level being filled by pumping a substantial amount (i.e., at least about 15%, and desirably above 50%) of the hydrocarbon therein at a level below the hydrocarbon-air interface therein. In prior methods of filling such storage structures, the liquid hydrocarbon was pumped to the top of the structure and filling effected from there. This involved unnecessary work since all the hydrocarbon was stored at a lower level. The process of this invention minimizes the amount of work needed to fill the structure by lifting the hydrocarbon only the minimum amount needed to effect filling and can reduce the needed work 50 to 70%.

This invention thus further provides the method of emptying an offshore hollow storage structure, which is partially above and partially below a sea level, containing a liquid hydrocarbon less dense than water which comprises emptying the storage structure of hydrocarbon therein above the sea level by gravity fiow or pumpinduced flow, and thereafter or simultaneously therewith withdrawing from the storage structure hydrocarbon stored therein substantially below the sea level by pumping water into the storage structure below the hydrocarbon-water interface, or at least below the level of the hydrocarbon being removed. The water pumped into the storage structure should not be raised higher than needed to avoid unnecessary work. Thus, the maximum height of the pumped water being delivered to the storage structure can be maintained at about or below the level at which the hydrocarbon is removed.

The storage structure 20a or other structure is anchored to a foundation resting on the bed 52 of the water body 54. In the illustrative embodiments, suitable excavations 96 are prepared in the bed and concrete bodies 98 are poured in the excavations and attached to rock bottom to provide the foundation. Alternatively, large concrete blocks may be employed in place of the concrete bodies 98 and rest on top of the bed, if the supporting ability of the bed is adequate. In a further alternative, the foundation may be provided by piles driven into the bed.

The supporting end portions 94 of the pedestals are imbedded in the concrete bodies 98, to support the structure 20a rigidly. In the embodiment of FIG. 14, the supporting end portions 94 constitute discrete portions of the pedestals 46a. The end portions are detachably connected to the remainder of the pedestals. The bottom end of 46a is permanently sealed independently of 94 to eliminate the need for a sealed connection by suit-able attachment or coupling joint structures 100. With this construction, the end portions may be present in the concrete bodies 98, after which the remainder of the storage structure 20a is attached thereto by the joint structures 100. Should it be desired to move the structure 20a to another location, the main body of the structure is disconnected from the end portions 94 and removed. The structure may be installed in like manner at the new location. Alternatively, the storage structure 20a may be detachably secured to a foundation in other ways.

Because of their nature, the structures provided herewith lend themselves well to a detachable connection to the foundation. The design and operation permit the existence of connections to the foundations which transmit only compressive forces; that is, the superstructure always presses down upon the foundation, and no tension or bending stresses are present. This occurs due to the weight of the structure and the elevated liquid load. A detachable connection thus offers both construction economies since the foundation can be installed and the entire structure floated to position and connected with minimum delay, and the advantage of ease of removal for relocation.

As illustrated in FIGS. -10 and 13, a service platform 102, 104, 106 or 108 may be mounted on the pedestals 46, or other pedestals, above the water level. Transfer, processing, and other equipment may be mounted on the platforms. Personnel facilities and space and facilities for a heliport may be provided on the platforms. The platforms are internally mounted on the pedestals and extend therebetween. The platforms preferably are mounted on or above the uppermost horizontal struts 88.

In further embodiments, illustrated in FIGS. 11 and 12, a plurality of storage structures 20 and 24, 20 and 22, or other structures, is employed, and a platform or 112 is mounted on the pedestals 46 of the respective structures to extend between the structures. While two structures are illustrated in each view, it will be apparent that more structures may be provided, and one or more platforms may be extended between two or more structures. As illustrated in FIG. 11, the internal platform 106 also may be provided on the storage structure 24. Similarly, internal platforms may be provided on one or more of the remaining structures 20 and 22, or on other structures as may be employed.

The elevated storage tanks and the platforms on the structures are supported by the pedestal-s above the reach of the highest waves. The pedestals braced by the struts 88 and 90, and also by the hollow body 92, are well adapted to withstand wave action. The storage tank system is durable and will withstand the most severe storms without interruption of its functions. The structure is rigidly fixed in place for maximum safety, and when employing the structure of FIG. 14 or the like, the structure can be removed and reused at another location at minimum expense.

While certain preferred embodiments of the storage tank system of the invention have been illustrated and described, it will be apparent that various changes and modifications may be made in the construction and arrangement of the components of the system within the spirit and scope of the invention. It is intended that all such changes and modifications be included within the scope of the appended claim.

What is claimed is:

1. An offshore storage tank system which comprises a plurality of at least three self-supporting storage tank units,

each of said units including a hollow pedestal for supporting the unit on the bed of a body of water and a storage tank mounted on and integral with the pedestal, said pedestals being in fluid communication with the interior of said storage tanks thereon and forming a single storage volume in each storage tank unit and being high enough to support the storage tanks above the level of the body of water,

a plurality of struts interconnecting said pedestals into a unitary storage structure,

a discrete detachable support member at the lower end of each pedestal for supporting the storage tank units on the bed of the body of water,

means for detachably connecting said support member to the pedestal, and

an externally sealed hollow body mounted on the lower part of the pedestals but above the detachable support member and below the storage tanks so that the hollow body is below the water level when the tank system rests on the bed of the body of water, whereby said structure is adapted to withstand the natural forces encountered, and said hollow body can impart buoyancy to said structure for moving the structure in the water.

References Cited UNITED STATES PATENTS 2,586,966 2/1952 Kuss et a1. 61--46.5 X 2,857,744 10/1958 SWiger et a1. 61-465 3,008,599 11/1961 Young 61-48 X 3,145,538 8/1964 Young 61-46 3,145,539 8/1964 Estes et a1. 6146.5

JACOB SHAPIRO, Primary Examiner.

U.S. Cl. X.R.

l37-10, 236; 220--l, l3 

